High-quality C/C ++ programming guide-expressions and basic statements

This series of articles is a summary of the key words "high quality programming guide-C ++/C language ".
This article summarizes the expressions and basic statements:

Operator priority:
Priority

Operator

Name or meaning

Usage

Integration direction

Description

1

[]

Array subscript

Array name [constant expression]

Left to right

 
()

Parentheses

(Expression)/function name (parameter table)

 
.

Member selection (object)

Object. member name

 
->

Member selection (pointer)

Object Pointer-> member name

 
2

-

Negative Operator

-Expression

Right to left

Single Object Operator

(Type)

Forced type conversion

(Data Type) Expression

 
++

Auto-increment operator

++ Variable name/variable name ++

Single Object Operator

--

Auto-subtraction Operator

-- Variable name/variable name --

Single Object Operator

*

Value Operator

* Pointer variable

Single Object Operator

&

Bitwise operators

& Variable name

Single Object Operator

!

Logical non-Operator

! Expression

Single Object Operator

~

Bitwise Inverse Operator

~ Expression

Single Object Operator

Sizeof

Length Operator

Sizeof (expression)

 
3

/

Division

Expressions/Expressions

Left to right

Binary Operators

*

Multiplication

Expression * Expression

Binary Operators

%

Remainder (Modulo)

Integer expression/Integer expression

Binary Operators

4

+

Add

Expression + expression

Left to right

Binary Operators

-

Subtraction

Expression-expression

Binary Operators

5

<

Move left

Variable <expression

Left to right

Binary Operators

>

Right Shift

Variable> Expression

Binary Operators

6

>

Greater

Expression> Expression

Left to right

Binary Operators

> =

Greater than or equal

Expression> = expression

Binary Operators

<

Less

Expression <expression

Binary Operators

<=

Less than or equal

Expression <= expression

Binary Operators

7

=

Equal

Expression = expression

Left to right

Binary Operators

! =

Not equal

Expression! = Expression

Binary Operators

8

&

Bitwise AND

Expressions & Expressions

Left to right

Binary Operators

9

^

Bitwise OR

Expression ^ expression

Left to right

Binary Operators

10

|

By bit or

Expression | expression

Left to right

Binary Operators

11

&&

Logic and

Expressions & Expressions

Left to right

Binary Operators

12

|

Logic or

Expression | expression

Left to right

Binary Operators

13

? :

Conditional Operators

Expression 1? Expression 2: expression 3

Right to left

Three-object Operator

14

=

Value assignment operator

Variable = expression

Right to left

 
/=

Assignment After Division

Variable/= expression

 
* =

Assign value after Multiplication

Variable * = expression

 
% =

Assign value after modulo operation

Variable % = expression

 
+ =

Add and assign values

Variable + = expression

 
-=

Value after subtraction

Variable-= expression

 
<=

Value after left shift

Variable <= expression

 
>>=

Value after right shift

Variable> = expression

 
& =

Bitwise AND postvalue

Variable & = expression

 
^ =

Value Based on bitwise XOR

Variable ^ = expression

 
| =

Value by bit or after

Variable | = expression

 
15

,

Comma Operator

Expressions, expressions ,...

Left to right

Sequential operation from left to right

Source of the operator priority table
Operators are not hard to remember. Priority and associativity are hard to remember!
Therefore, we recommend that you use parentheses to determine the Operation Sequence of expressions if there are many operators in the code line. You can avoid using the default priority, especially when you forget the priority.

If statement (compared with zero value)
Comparison between Boolean variables and zero values:
Boolean variables cannot be directly compared with TRUE, FALSE, or 1 or 0.
The correct method is as follows:
[Cpp]
If (flag) // indicates that the flag is true.
If (! Flag) // indicates that the flag is false.
// The following is a bad style
If (flag = TRUE)
If (flag = FALSE)
If (flag = 1)
If (flag = 0)

Comparison between Integer Variables and zero values:
Integer variables should be replaced with "=" or "! = "Direct comparison with 0
The correct method is as follows:
[Cpp]
If (value = 0)
If (value! = 0)
// The following write method can be misunderstood as a Boolean variable.
If (value)
If (! Value)

Comparison between floating point variables and zero values:
You cannot use "=" or "! = "Compare with any number
Because the floating point type has precision restrictions, you should try to convert it to the "> =" or "<=" Form
Therefore
[Cpp]
If (x = 0.0)
Convert
[Cpp]
If (x> =-EPSINON) & (x <= EPSINON) // Where EPSINON is the allowable error (accuracy)

Comparison between pointer variables and zero values:
Use "=" or "! = "Comparison with NULL
Although the value of NULL is the same as that of 0, they have different meanings.
[Cpp] view plaincopyprint?
If (p = NULL)
If (p! = NULL)
// The following statement is confusing: integer variable
If (p = 0)
If (p! = 0)
// The following statement is a Boolean variable.
If (p)
If (! P)
Sometimes you may see the if (NULL = p) Strange format. This is not a program error, but a programmer wants to prevent the if (p = NULL) if (p = NULL ). The compiler considers if (p = NULL) as legal, but it indicates that if (NULL = p) is incorrect because it cannot be assigned a value.

Efficiency of loop statements
Suggestion:
In multiple loops, if possible, we should try our best to put the longest loop in the innermost layer and the shortest in the outermost layer. To reduce the number of times the CPU cross-switch cycle layer
If logical judgment exists in the loop body and the number of cycles is large, it is recommended to move the logical judgment outside the loop body.
You cannot modify the loop variable in the for loop to prevent the for loop from being out of control.
We recommend that you use the "half-open and half-closed" Method for the value of the for statement's cyclic control variable.

Switch statement
Break must be added at the end of each case statement unless multiple branches are intentionally overlapped.
Do not forget the last default Branch, which must be retained even if the program does not need it. This is to prevent others from mistakenly thinking that you forgot to use default to process www.2cto.com.

Goto statement
Although goto will undermine the structural design style. It may skip the construction of some objects, initialization of variables, and important computing statements, resulting in errors or risks. Therefore, goto is generally not recommended. However, one of its functions cannot be ignored: It can jump from multiple loops to the outside of the loop at once without multiple break statements.